Aluminum barrels (e.g., cylinders) are weldable to domes (e.g., hemispheres) to make launch vehicle propulsion or fuel tanks. Such barrels may be made from four 90.degree. curved panels or five 72.degree. curved panels that are weldable together along longitudinally extending seams in a vertical weld fixture or tool. Generally, for purposes of welding the panels to form a fuel tank, the panels may be placed on a horizontal turntable of the vertical weld fixture and then rotated about a vertical axis into clamping fingers of the vertical weld fixture. Thereafter, a one or two torch-single pass/variable polarity gas tungsten arc (VPGTA) system may be used to fusion-weld butt jointed panels together. For a four panel fuel tank, this process is repeated four times to produce a complete barrel, and five times for a five panel fuel tank.
After welding, the barrel is removed from the vertical weld fixture and transported to a weld inspection area. This is a very cumbersome and labor intensive process due to the size and weight of the completed barrel. It may take several riggers several hours to transport the barrel. The quality requirements for the weld joints are very high due to the high pressures and variable temperatures to which launch vehicle propulsion tanks and fuel tanks are exposed. Types of undesirable defects that may be encountered in the welds during inspection may include porosity, cracks, oxide inclusions, burn through holes, and lack of fusion. Factors which can contribute to such defects include the temperature, humidity, and barometric pressure at the time the weld is performed, along with oxidation, alloy variations, and imperfections of the weld wire itself.
There are several well known weld inspection methods for revealing possible weld defects. One basic method is X-ray photographing. Typically, in X-ray photography, X-ray film is placed on one side of the weld seam and an X-ray source is placed on the other side. The X-ray film is then exposed by the X-ray source through a portion of the weld. The X-ray source and a new X-ray film are then moved to a next position along the weld seam, and the next portion of X-ray film is exposed. This process is repeated until 100% of the entire length of the weld seam has been X-ray photographed. The X-ray film is then developed and reviewed by an inspector. Any defects found by the inspector are noted, located, and marked on the weld seams. The barrel is then repositioned in the vertical weld fixture and the marked areas are rewelded. The barrel is once again removed from the vertical weld fixture, placed in the weld inspection area, and the rewelded portions of the weld seam are X-rayed again. Upon developing and reviewing the X-ray film, if the defects have not been corrected, the barrel is returned once more to the vertical weld fixture for further rewelding. This process is very time consuming and labor intensive.
When thicker barrel material is used, due to penetration limitations of current film-based X-ray devices and the difficulty of detecting defects in aluminum, multiple films may be required to image the full range of thickness for each portion of the weld to adequately inspect the weld. In such cases, for each section of the weld, a first X-ray film is exposed for the first part of the thickness of the weld, and a second X-ray film is exposed for a next part of the thickness of the weld, and so on. This limitation significantly increases the time it takes to inspect variable thickness weld seams.